1. Field of the Invention:
The present invention relates to a drive circuit for a Langevin type ultrasonic bolt-tightening motor.
2. Description of the Related Art:
The Langevin type ultrasonic bolt-tightening motor is desired for practical use since it was silent in operation and had a simplified structure. Further, such an ultrasonic motor has many other features, such as being capable of operating at a speed higher than that of the traveling-wave type ultrasonic motor.
The Langevin type ultrasonic bolt-tightening motor comprises a stator section including a piezo-electric element to which a motor drive voltage is applied and a rotor section is rotationally driven by longitudinal and torsional vibrations resulting from the application of the motor drive voltage to the piezo-electric element. The resulting torque depends on the frequency of the motor drive voltage. The maximum efficiency of generation of the torque is accomplished at a particular frequency (hereinafter called "optimum drive frequency").
The optimum drive frequency is inherent in a particular motor and has a very narrow band width. The optimum drive frequency is not always constant in operation and continuously varies depending on changes in various factors such as temperature in the element, magnitude of the load and the like.
The prior art could not electrically detect the variable optimum drive frequency for feedback control. In order to correct such a defect, the prior art should maintain the frequency of the motor drive voltage constant at all times or set it at a value near a frequency which appears to be the optimum drive frequency. Thus, the prior art could not feedback control the frequency of the motor drive voltage following the variable optimum drive frequency.
Therefore, the Langevin type ultrasonic bolt-tightening motor constructed in accordance with the prior art cannot perform efficiently since the generation of the drive torque is not accurately controlled.